There are several types of joints in the human body. These can be categorized into weight bearing and non-weight bearing joints. The hip, knee, ankle and intervertebral disc in the spine are considered load-bearing joints, while the finger and toe are considered non-weight bearing joints. The hip, knee, and ankle are further categorized as synovial joints, while the intervertebral disc is a cartilaginous joint. These joints, especially the weight bearing joints, can undergo degenerative changes due to disease, age, trauma, repetitive loading and/or genetics.
For synovial joints, these degenerative changes come in the form of arthritis, or inflammation of the joint, leading to damage of the articular cartilage. Osteoarthritis mainly damages the joint cartilage, but there is often some inflammation as well. Rheumatoid arthritis is mainly inflammatory, and can eventually destroy the joint cartilage and adjacent bone. Fracture and other forms of trauma may also lead to degenerative changes. Osteonecrosis is a condition in which either the bone of the femoral head or femoral condyles dies. The dead bone cannot withstand the stresses of walking and as a consequence, the femoral head or condyles then collapse, become irregular in shape, and cause pain in the hip or knee joints.
The spinal intervertebral disc, which is situated between the vertebral bodies, is also subject to degenerative changes. The spinal disc is comprised of a tough outer ring call the annulus, and a jelly-like filling call the nucleus. Degeneration of the intervertebral disc can occur at a relatively young age, and is believed to be the major cause of low-back pain. It often begins with a structural change in which the nucleus loses its water-binding capacity and the disc consequently loses height. Typically this is manifested by desiccation of the nucleus. After this happens, more compressive loading shifts to the annulus, rendering this structure more susceptible to delamination and damage. This in turn can lead to fissures in the annulus with the possibility of a corresponding herniation of nucleus material. This leads to a decrease in the intervertebral disc height, encroachment upon the nerve roots and/or spinal canal and degeneration of other surrounding tissues.
The individual whose joints undergo such changes may incur significant discomfort, pain and even disability. Initially, the only option for the patient with degenerative changes to these joints was to undergo arthrodesis, or fusion, of the effected joint. Although this can effectively relieve pain and lead to an increase in the quality of life, fusion can significantly alter the normal biomechanical function of the effected joints. Treatment options have since advanced to include motion preserving implants, known as arthroplasty devices. These joint replacement devices usually comprise a pair of endplates with some type of intermediate components or articulating bearing surfaces to facilitate motion between the adjacent vertebral bodies.
Artificial knee joints have been provided which comprise a femoral component having a pair of metal condyles at the distal end and a tibial baseplate including a plastic articulation component mounted to the baseplate. The condyles articulate against the tibial articulation component to provide joint mobility. Known artificial hip joints typically include a stem inserted into the proximal femur, a generally spherical head mounted to the stem, and an acetabular cup housing a plastic wear liner. This creates a ball and socket type connection mimicking that of the natural hip.
In the spine as well, a primary surgical treatment for disc degeneration has been fusion. However, spinal fusion has proved to cause an increase in degeneration at other vertebral levels that must compensate for the loss of motion at the fused level, commonly causing the patient to relapse into more pain and limited mobility. Further advances have included the development of motion preservation intervertebral disc replacement devices. Such devices typically comprise a pair of biocompatible metal plates having outer surfaces for engaging the inferior and superior vertebral surfaces, and opposed inner articulating surfaces to support multi-axial motion. The inner surfaces of the plates may be lined with a hard polymer articulating core, or the plates may house an elastomeric core. Other known devices comprise a pair of metal plates with inner metal-on-metal articulating surfaces.
A key challenge for arthroplasty devices, whether for the hip, knee, ankle or spine, is selecting the proper materials for the various components thereof. Biocompatibility—the suitability of a material for exposure to the body or bodily fluids—and biodurability—the ability of a material to maintain its physical and chemical integrity after implantation in to living tissue—are essential for permanent medical implants. Materials chosen should avoid cytotoxicity, systemic toxicity, irritation, macroscopic or allergic reactions, muscle degeneration, or other adverse response. The biocompatibility and biodurability requirements significantly limit the selection of materials available for weight bearing devices.
The implant components must also exhibit sufficient strength and excellent fatigue performance to avoid mechanical failure over a long life under physiological loadings and kinematics. Properties such as yield strength, break strength, flexural strength, shear strength, and compressive strength of the implant components can significantly impact the success of the implant in weight bearing joint arthroplasty. Hard and stiff materials, such as ceramics or metals, have favorable strength characteristics. However, such materials have substantially higher flexural modules than that of cortical bone. This can cause a phenomenon known as “stress shielding,” which may cause bone loss and the loosening and eventual failure of the implants. Certain polymer materials, having a flexural modulus similar to cortical bone, are thought to minimize stress shielding and the associated adverse effects. However, many polymers do not have sufficient yield strength to be used in weight bearing joints. One proposal has attempted to address these considerations by providing an intervertebral implant having outer plates made of a softer polymeric material to interface with the vertebral bone faces, and a hard ceramic or metal material for the inner articulating components. However, such a design is more complex to manufacture and raises further challenges in the need to inhibit slight motion between the two dissimilar materials combined in a single component of the multiple component implant to minimize wear and ultimate separation at those interfaces.
The problem of stress shielding also exists in other implants such as knee and hip replacements. In an artificial hip, for example, stress shielding may occur where a metal acetabular shell is secured directly to bone, and where the femoral stem is retained in the proximal femur. In a knee implant, stress shielding may cause bone degradation where a metal femoral condyles meet the distal end of the femur.
As exemplified in the devices described above, known hip, knee and ankle arthroplasty devices, and the majority of disc arthroplasty devices, incorporate articulation in their design. The articulation can be conforming, such as the ball and socket arrangement of the hip joint, or non-conforming, which permits sliding motion such as in known knee arthroplasty designs. In both conforming and non-conforming designs, the motion of the articulation surfaces against each other generates wear particulate. The primary wear that occurs in a hip prosthesis is between the femoral head and the acetabular cup. In a knee prosthesis, wear occurs primarily between the distal femoral condyles and the articulation surface of the tibial tray. The generation of wear particulate is important not only from a device lifetime perspective, but also from a biological perspective. In some cases, the biological response will dictate the lifetime of the device. This is because the generation of wear particulate in sufficient amount and size may lead to an adverse cellular response, manifested by macrophage activation, giant cell formation and a cascade of cytokine release ultimately leading to an imbalance in osteoclast and osteoblast activity. This may lead to inflammation of the tissue around the reconstructed joint, osteolysis and failure of the implant.
The wear performance of such devices is a function of the mechanical design as well as the relative material properties of the articulating components. For example, the degree and types of motion permitted, the contours of the articulating surfaces, contact geometry, speeds, loads, micro-motion between integrated components, surface roughness and lubricant and other factors can significantly impact the amount and nature of the wear debris. The material selection and device design must result in acceptable wear performance of the device under expected physiological loadings and kinematic conditions.
Early attempts to evaluate the wear properties of these devices typically involved the use of small scale testing apparatuses such as pin-on-disc or pin-on-plate configurations. A pin-on-disk wear simulator consists of a disk that is mounted to and driven by a turntable at constant speed in an environmental chamber. The disc is made from the metal, ceramic or polymer representing the first bearing surface. Test pins formed of the second material of the bearing couple are loaded against the turntable by either static weights or a hydraulic system. The mass loss of the pins may be determined as a function of sliding distance, and a wear factor calculated. This type of simulator likely underestimates the amount of wear that can occur when evaluating materials for orthopedic weight bearing applications, since it does not depict the actual kinematics of the joints.
Reciprocating pin-on-plate testing machines may better approximate the oscillatory sliding motion of synovial joints. The pins in such devices are similar to those used in pin-on-disk testing. However, the pins rotate while being statically loaded against a plate that oscillates back and forth. The mass loss of the pins may be determined as a function of sliding distance and the wear factor calculated. This type of simulator is believed to provide a more accurate evaluation of the wear performance of biomaterials in weight bearing joint applications.
An even more realistic prediction of clinical wear performance may be obtained using hip, knee and spine simulators. Hip and knee simulators are able to apply the necessary dynamic loads and the complex motions experienced in the clinical setting. These devices can produce the multidirectional motion profiles necessary to generate wear rates and wear particle morphologies that are similar to in vivo experience. There are currently several published regulatory standards and guides known to those skilled in the art for properly evaluating material combinations to be used in orthopedic weight bearing environments. Simulator testing consists of mounting the femoral head and the acetabular shell of the hip prosthesis, or the femoral condyles and tibial tray of the knee prosthesis, on the simulator and bathing it in a protienacious solution that is thought to mimic the synovial environment to which it will be exposed to in vivo. The simulator generates the loadings and motions of a typical gait profile. The duration of the test is generally from 5 to 10 million cycles of gait simulation or until a steady state wear is observed. One million cycles of testing is thought to represent the amount of walking the average person undergoes in one year. The test is stopped at predefined intervals, and the mass loss of the articulating components is measured. The wear rate is then determinable from the mass loss in a known manner.
Simulators that attempt to mimic the loading and range of motion of the intervertebral disc are also known in the art. However, the intervertebral disc is a more complex joint than that of the hip and knee, and there is presently no established protocol for assessing the wear in such device. The intervertebral disc has a complex multidirectional motion profile including rotational movements in the fore-and-aft direction and the lateral direction as well as rotation about the vertical axis. However, the fore-and-aft rotary motion component of the multidirectional motion profile of the intervertebral disc is the predominant motion component. The hip joint has a similar motion profile it must be able to undertake. The knee joint undertakes substantially unidirectional motion as by pivoting or rotational movement in the fore-and-aft plane. Nevertheless, whereas total joint simulator testing for the hip and knee has evolved to the point where there are several published standards by regulatory bodies for evaluating material combinations for artificial hip and knee joints, as mentioned it is generally recognized that the intervertebral disc is a more complex joint than that of the hip and knee with the load and kinematic profiles not being well understood.
The materials of fabrication also impact other important aspects of total joint arthroplasty devices beyond wear performance. For example, metal surfaces tend to generate scatter which prevents a complete inspection of tissue and bone growth using conventional imaging techniques such as X-rays, MRI and Computer Tomography. Radiolucent materials, such as polymer materials, generally do not interfere with the imaging of the surrounding bone and tissue using these techniques. Material selection may also impact required sterilization techniques, as certain biopolymers are known to oxidize when sterilized using conventional techniques such as steam sterilization. This may adversely affect the strength and/or the wear performance of the material.
A number of attempts to utilize polymer against metal bearing couples in weight bearing joint replacement device have been reported in the art. The relatively harder metal components can provide strength and durability to such devices, while the relatively softer polymer provides for a low friction interface. The polymeric materials also wears before the metal to minimize the generation of potentially harmful metal ions. However, many of the biopolymers investigated have been found to have insufficient yield strengths, wear resistance and/or biocompatibility and biodurability to succeed in weight bearing joint applications. And the requirement of metallic structural components creates additional drawbacks, such as interference with imaging and the previously mentioned phenomenon of stress shielding. Nevertheless, the use of polymer on metal bearing interfaces in such devices has met with some success, as described in more detail below.
Early hip replacement implants used Polytetraflourethylene (PTFE) as a bearing material against metal. Such devices typically included a stainless steel femoral head articulating against an acetabular cup made of PTFE. The PTFE provided a low coefficient of friction at the bearing interface, but its low yield strength and lack of durability purportedly led to excessive wear and severe inflammation of the periprosthetic tissue in clinical use. The use of glass filled PTFE and a mica filled PTFE has been shown to improve the wear rates when evaluated on a pin-on-plate wear tester. However, it has been reported that this did not translate to reduced wear rates clinically, and resulted in severe wear like that of unfilled PTFE.
The use of ultra-high molecular weight polyethylene (UHMWPE) against metal in total joint replacements has a long clinical history dating back decades. UHMWPE was proposed as a counterface to stainless steel due to its greater biocompatibility and increased wear resistance over PTFE when evaluated on pin-on-plate wear testing simulators. UHMWPE also possesses superior mechanical toughness and wear resistance over most other polymers. UHMWPE on metal hip joints have succeeded clinically, with high rates of survivorship beyond 25 years in some cases. However, UHMWPE is also known to have certain drawbacks and limitations. These include the need for small diameter head sizes to reduce the frictional torque due to less than optimal lubrication, oxidation of the UHMWPE resulting from ionizing sterilization, and wear caused by third body debris such a bone particulate.
A significant drawback of UHMWPE is the accumulation of wear debris eliciting an adverse cellular response leading to inflammation and osteolysis of the surrounding bone. The literature suggests a threshold wear rate of 80 mm3/year, above which particle induced osteolysis may lead to failure. The clinical wear rate of UHMWPE hip implants can potentially exceed this value. It has been suggested that the UHMWPE wear volume can be controlled below the indicated threshold for osteolysis by limiting the diameter of the femoral head. However, a smaller head decreases the range of motion of the joint and elevates the risk of the neck of the femoral stem impinging upon the cup causing dislocation of the femoral head.
Another potential drawback of devices utilizing UHMWPE on metal bearing interfaces is the potential oxidation of UHMWPE. This has been shown to occur as a result of using gamma radiation or electron beam radiation for sterilization in an oxygen rich environment, and the subsequent storage of the polymer in air or exposure in vivo. Oxidation of UHMWPE causes the polymer to become more brittle, decreases the fatigue strength fracture resistance of the material, and makes it less biocompatible. It is believed that UHMWPE oxidation also decreases the wear resistance of the material. To avoid these problems, sterilization of UHMWPE implants must be performed in an inert environment, such as nitrogen.
The performance of UHMWPE on metal joint implants may also be adversely impacted by third-body wear particulate. For example, cements such as Polymethylmethacrylate (PMMA) are commonly used to secure the metal femoral stem of a hip prostheses into the femoral canal or the metal backing of the tibial tray to the tibial canal. PMMA particles can become entrapped between the head and UHMWPE acetabular cup. Such third-body wear particulate can also comprise bone or metal particles. This may lead to accelerated wear of the UHMWPE in such bearing couplings, either as a result of the abrasive effect of the particulate on the UHMWPE surface and/or by roughening the surface of the metal head bearing surface.
Attempts have been made to improve the wear resistance of UHMWPE on metal bearing couples by reinforcing the UHMWPE with carbon fibers. It has been suggested that carbon fibers would increase the modulus and the ultimate tensile strength of the UHMWPE, and decrease its creep properties, resulting in improved wear performance. However, reported simulator testing has been inconclusive, with some studies reporting less wear than conventional UHMWPE and others showing high wear rates. Carbon fiber reinforced UHMWPE on metal knee implants have not achieved clinical success. The clinical failure of this bearing couple is thought to be due to the carbon fibers being mechanically but not chemically bonded to the UHMWPE, the poor creep resistance of the UHMWPE which promoted debonding of the carbon fibers from the matrix, the tendency of the carbon fibers to scratch the metal counterface, and the high stress loadings of the knee joint.
Another drawback of UHMWPE on metal weight bearing joint implants is the potential for increased wear over time as a consequence of a cross-shearing effect on the UHMWPE. Conventional UHMWPE undergoes a molecular re-organization process under both unidirectional and multidirectional motion.
Normally, the lamellar structure of UHMWPE is randomly arranged. Under unidirectional motion, the shear and tensile stress vectors applied to the polymer chains within the wear surface cause the polymer chains to become oriented in the direction of the stress vector. This results in a strain hardening of the polymer and an increase in the wear resistance in the direction of travel as this motion continues. However, the strain hardening of the polymer in one direction leads to weakening of the polymer in the perpendicular direction. Multidirectional motion, consisting of tensile and shear stresses in multiple directions, thus results in strain hardening in the primary flexion/extension direction and the subsequent softening of the polymer in an off axis direction. It is believed that strain hardening of UHMWPE increases the long term wear of implants subjected to the complex multidirectional motion of weight bearing joints, and particularly with motion preservation intervertebral implants. Strain hardening also creates further challenges by making it more difficult to predict the clinical performance of the device through joint simulator testing.
Significant reductions in wear rates against metal have been realized by crosslinking the polymer chains that comprise UHMWPE. Gamma or electron beam radiation is used in various doses and with different types of remelting or annealing thermal treatments to reduce or eliminate residual free radicals. Crosslinking may also be performed via peroxide or silane chemistry. These techniques require further processing steps, add cost, and make control of material properties more difficult. Crosslinking of UHMWPE may lead to undesirable material changes such as decreased tensile and ultimate strength, and also a decreased elongation to failure. The addition of thermal treatments can make the polymer still susceptible to oxidation and reduce its fatigue strength, which could adversely impact clinical performance.
Despite the above drawbacks, total weight bearing joint arthroplasty devices utilizing UHMWPE on metal articulation couples, which originated in the hip and knee applications, have enjoyed moderate clinical success and are well accepted in the art. It is believed that the development of disc arthroplasty devices has benefited greatly from the established clinical history and lessons learned from the technological evolutions in the hip and knee arthroplasty industry. Although osteolysis remains a major concern of using UHMWPE, it has been used in several available total disc replacement implants since it is a proven material combination supported by an extensive clinical history. These devices typically consist of a UHMWPE core that has been moderately crosslinked articulating against Cobolt-Chromium (CoCr) endplates. The wear resistance of UHMWPE and its overall durability remains a concern, with some published wear rates being similar to those seen for hip arthroplasty. The high wear rates are a source of concern because disc arthroplasty devices are indicated for a much younger population, and the goal is to have such a device last the patient lifetime. Nevertheless, these devices have been used clinically with relatively few reported complications related to the UHMWPE material reported to date. However, they nevertheless suffer from the potential drawbacks of stress shielding and interfering with imaging associated with metal components, as discussed above.
Other polymer on metal bearing couples in weight bearing joint replacements have also been investigated. Thus, for example, one known hip replacement device utilized polyacetal articulating against a stainless steel femoral head. This device also incorporated a sleeve of polyacetal between the neck of the femoral stem and the metal ball. This prosthesis suffered from a high failure rate due to excessive wear of the polyacetal components. Osteolysis was also more prevalent than observed in UHMWPE hip replacements. Friction in retrieved polyacetal acetabular cups has been shown to be significantly greater than in retrieved polyethylene acetabular cups. These frictional characteristics of polyacetal can change as the material ages in vivo, making the long term performance of the material uncertain.
PEEK, or polyetheretherketone, is an engineering thermoplastic used in certain medical implant applications. It is available from at least one manufacturer, Invibio Ltd. of Lancashire, UK, in pure form and also in other formulations containing additives such as carbon fiber, barium sulphate and glass fiber. Carbon fiber reinforced PEEK (CFR-PEEK) and glass fiber reinforced PEEK include short carbon or glass fibers that increase the strength of the polymer for higher stress demanding applications. The material is also available as a composite comprising PEEK as a matrix polymer in combination with continuous carbon fibers for applications requiring even greater strength and rigidity. As used herein, the terms “PEEK material,” or “PEEK-type material” are intended to include all materials of the polyaryletherketone family such as PEEK (Polyetheretherketone), PAEK (polyaryletherketone), PEK (polyetherketone), PEKK (polyetherketoneketone), PEKEKK (polyetherketoneetherketoneketone), PEEKK (polyetheretherketoneketone), and PAEEK (polyaryletheretherketone). It should be noted that the material selected can also be filled. For example, other grades of PEEK are also available and contemplated, such as 30% glass-filled or 30% carbon filled, provided such materials are cleared for use in implantable devices by the FDA, or other regulatory body. Glass filled PEEK reduces the expansion rate and increases the flexural modulus of PEEK relative to that portion which is unfilled. The resulting product is known to be ideal for improved strength, stiffness, or stability. Carbon filled PEEK is known to enhance the compressive strength and stiffness of PEEK and lower its expansion rate. Carbon filled PEEK offers wear resistance and load carrying capability.
As will be appreciated, other suitable similarly biocompatible thermoplastic or thermoplastic polycondensate materials that resist fatigue, have good memory, are flexible, and/or deflectable have very low moisture absorption, and good wear and/or abrasion resistance, can be used without departing from the scope of the invention.
PEEK has been used to fabricate medical implants for fusion applications, such as spinal fusion cages, and in bone screws, pins and other applications not involving joint articulation. PEEK, like other polymers such as UHMWPE, has also been proposed as a material for articulation surfaces in self-mating bearing couples for non weight bearing joints such as a finger joint.
The use carbon fiber reinforced PEEK as an alternative to UHMWPE for articulation against a metal counterface in weight bearing arthroplasty devices has also been investigated. An assessment of the wear characteristics for CFR-PEEK articulating against CoCr using hip simulators showed a wear rate significantly lower than that of conventional UHMWPE on CoCr. However, it has been suggested that this material is not suitable for high stress environments such as in the knee joint. Regardless of the suitability of CFR-PEEK against metal in such applications, this bearing coupling still carries the above-described drawbacks associated with the use of metal structural and articulation components.
Certain cervical disc replacements are known to utilize polyurethane articulating against titanium. However, the wear resistance of that coupling is questionable, considering that the load encountered in the cervical region is roughly an order of magnitude lower than that of the lumbar region and the device has been reported to exhibits wear rates higher than the traditional UHMWPE on CoCr and metal on metal devices.
Early total joint arthroplasty devices utilizing metal on metal articulation surfaces were considered unsuccessful. Later attempts utilized the more wear resistant CoCr in place of stainless steel. However, the initial success rate proved to be unsatisfactory for reasons such as poor manufacturing tolerances, inadequate clearances, early impingement, poor material selection and increased failure rates due to high torsional forces, leading to loosening and accelerated wear. The introduction of cementing of the femoral shaft led to a decrease in the rate of loosening, and the success rate improved. However, because of their high initial failure rates and occasional dark staining of the periprosthetic tissues by metallic wear debris, these devices were largely supplanted by the acceptance of the metal on UHMWPE designs as the primary choice for use in total joint arthroplasty.
Even so, a large number of metal on metal hip prostheses have reportedly functioned successfully for more than two decades. The average volumetric wear rates are reportedly on the order of 10 to 200 times less than that typically seen in conventional UHMWPE bearings, with little or no biologic response to the wear process. It therefore appears that the problems undermining the clinical success of the first generation implants were primarily from sub-optimal implant design and not the inherent wear resistance of the bearing couple itself. Given the potential for greatly improved wear performance, further investigation into metal on metal hip arthroplasty devices has led to improved manufacturing techniques and material selection resulting in improved metallurgy, sphericity, surface finish and clearances. Reported hip simulator testing and clinical retrieval data suggest that these advances have further reduced the volumetric wear and improved clinical survivorship rates.
Perhaps due to the low wear characteristics and longer expected lifetime, metal on metal disc arthroplasty devices have also been pursued in the art. Although decreased wear rates are reported for these metal on metal bearings in comparison to CoCr on UHMWPE, the metal particles generated are smaller and greater in number than in a conventional UHMWPE bearing, and may present an overall active surface area similar to that of UHMWPE. These soluble metal particulates lead to elevated chromium, nickel and molybdenum levels and may result in hypersensitivity and potential toxicity, carcinogenesis and mutagenicity. Therefore, metal on metal bearings in spinal disc arthroplasty continue to suffer from the perception of elevated ion exposure potentially leading to elevated health risks, in addition to other drawbacks such as stress shielding of adjacent bone and interference with imaging of the surrounding tissue.
Further attempts to reduce wear rates and potentially alleviate the problem of osteolysis have included the pairing of biopolymers and ceramics in weight bearing joint replacement devices. Ceramic materials generally have increased hardness, the potential for a smoother surface finish, and are less susceptible to scratching compared to metals. Ceramics also provide better wettability compared to metals and therefore offer improved lubrication. Simulator studies of an artificial hip comprising a ceramic femoral head articulating against a UHMWPE acetabular cup demonstrated a very low wear rate for this bearing couple. However, the same reduction in wear rates has not been consistently demonstrated clinically in reported radiographic studies.
Another attempt paired CFR-PEEK as an acetabular cup articulating against a ceramic femoral head in a hip prosthesis. It has been reported that optimization of the percent carbon reinforcement results in a wear rate of the acetabular cup that is significantly less than that of conventional UHMWPE when paired with CoCr, alumina or zirconia. However it has also been suggested that that CFR-PEEK should only be used in a conforming bearing surface, such as an acetabular cup for a hip joint for supporting a metallic or ceramic femoral ball, and would not perform well as in a high stress, non-conforming contact situation such as a tibial component in a knee joint.
Still other investigators have reported on total joint arthroplasty devices comprising ceramic on ceramic articulation interfaces. As mentioned, ceramics are much harder than metal, resulting in increased scratch resistance, and they can also be manufactured to a much smoother surface finish. They are also hydrophilic, permitting a better wettability of the articulating surface. The improved wettability and finish of the articulating surface results in a fluid film that offers a reduction in the coefficient of friction as compared to metal on metal.
Early reported attempts utilizing alumina ceramic on ceramic bearings in hip replacements resulted in poor clinical results, with high wear or fracture resulting in high failure rates. Improved manufacturing and design has resulted in much lower reported fracture rates for current generation alumina femoral heads. Other devices have utilized zirconia, which has a significantly greater fracture strength and toughness than alumina. The suitability of zirconia as a self mating bearing couple is a subject of considerable disagreement in the art, with some investigators reporting severe wear, possibly as a result of thermal instability, and others reporting very low wear. More recent efforts have led to the development of alumina-zirconia composite ceramics for joint arthroplasty devices. This combines the high strength of zirconia with the thermal stability of alumina.
Ceramic on ceramic bearings have demonstrated the lowest in vivo and in vitro wear rates to date of any bearing combination. Ceramic bearings do not share the same biological concerns from generated particulate debris as metal bearings, as they are considered to be relatively biologically inert. However, ceramics are prone to material failure when subjected to high mechanical stress, either in tensile or impact loading, which may limit their long term potential total weight bearing joint arthroplasty.
Other weight bearing joint replacement devices have been proposed that utilize compliant bearing surfaces provided as coatings of metal structural components. For example, one known attempt involved the use of a compliant material as a surface covering of a metal femoral ball articulating against the native cartilage of the acetabulum. Materials for use have included silicon rubbers, polyurethanes and olefin based synthetic rubbers. These devices have been shown to operate with very low friction because of the fluid-film lubrication that they exhibit, and therefore should produce lower wear than current prosthetic materials as the two surfaces of the joint are completely separated by a film of synovial fluid. They have been shown to possess a balance of physical strength, flexibility, dynamic flexural endurance, inherent chemical stability and physiological compatibility. The use of elastomers such as polyurethane as an articulating weight bearing material have not shown any benefit in terms of wear resistance over the more traditional bearing couples, and this may lead to questions regarding their biodurability and subsequent biocompatibility.
Thus, as detailed above, the stringent requirements for permanent articulating weight bearing arthroplasty devices have restricted the possible material selection, material treatments and surface designs that can be used in clinically viable products. Because of these significant limitations, the predominating material couplings for the weight bearing arthroplasty devices are metal on UHMWPE, metal on metal, and ceramic on ceramic. Cobalt chrome self-mating devices have been shown to have superior wear resistance over UHMWPE, but elevated metal ions and delayed type hypersensitivity has been seen as a potential clinical issue. The potential carcinogenicity due to metal ion exposure has not been resolved. Metallic components also cause stress shielding effects and interfere with imaging. Ceramic on ceramic couples also exhibit high wear resistance, but they suffer from limited design options and may potentially fracture under physiological loadings. Other combinations have also been studied and undergone various degrees of development and implementation. Some of these material combinations have shown promise when tested in the laboratory, but many have also failed laboratory testing, and few have translated into clinical success.
Solutions to the drawbacks associated with the design and material selection of previously proposed arthroplasty devices have remained elusive. Few attempts to provide polymer against polymer bearing surfaces in weight bearing total joint arthroplasty devices have been reported. One attempt involved the use of polyacetal articulating against UHMWPE. In vitro comparisons using hip simulators of polyacetal articulating against UHMWPE with UHMWPE articulating against CoCr showed lower friction and wear in the all polymer combination. Although formaldehyde, a product of the degradation of polyacetal degradation, was present in trace amounts in some lubricant samples, the initial study demonstrated the potential of this bearing couple for clinical applications. Reported results of a subsequent clinical trial using a knee prosthesis consisting of a polyacetal femoral component bearing against a conventional UHMWPE tibial tray suggested performance comparable to metal on conventional UHMWPE implants. Recovered implants demonstrated only minor signs of wear and biological activity. However, sterilization of polyacetal with gamma radiation in air results in a material change, manifested by a change in color. Also, cementless fixation of the polyacetal component has been reported to be inadequate, resulting in high rates of aseptic tibial loosening and infection. As a result, polyacetal has not been considered a viable alternative to using metal as the counter bearing material to UHMWPE.
The use of UHMWPE self-mating bearing couplings, i.e. where both articulating components have UHMWPE bearing surfaces, is also considered infeasible in weight bearing joints due to the low yield strength and high wear associated with this material. Instead, the use of polymeric self-mating bearing couples has been confined to non-load bearing joints such as the finger. In this regard, it has been noted that cross-linked polyethylene (XLPE) can be used as a self-mating bearing couple in the finger since the wear rate, although approximately six times greater than for non-cross linked UHMWPE against stainless steel bearing couple, still provides sufficient wear resistance for applications with low loading, such as the metacarpo-phalangeal joint of the finger.
Apparently due to the known limitations of common biopolymers discussed above, such as low yield strength leading to adhesion deformation, high wear caused by third body particulate, cross shearing and/or oxidation, the path of development of arthroplasty for weight bearing joints has largely bypassed polymer on polymer bearing couples. Indeed, many of the biopolymers investigated were found to have insufficient strength, wear resistance and/or biocompatibility to succeed in weight bearing joint applications when paired against metal or ceramic components. Devices that have incorporated polymer bearing materials with some success, namely UHMWPE or polyurethane, have also incorporated much harder metal or ceramic components to serve as the major structural members and as bearing counterface materials for mechanical strength and wear performance. Thus, all polymer combinations have generally been excluded as an acceptable self-articulating material combination for use in weight bearing joint replacements.
There is in particular an extensive history of failure when polymers articulate against polymers in weight bearing artificial joints. PEEK has been suggested as an appropriate structural material for use in medical implants due in large part to its strength, radiolucent nature, and biocompatibility. The primary applications of this material have been in structural implants having no articulating component. A PEEK on PEEK articulation interface for non-weight bearing joints such as in finger joints has been proposed by one investigator. This is consistent with the prior art teachings that self-mating polymeric bearing couples should not be employed in load bearing joints. However, the prior art lacks any suggestion of weight bearing joint arthroplasty devices comprising PEEK as a primary structural material and having PEEK on PEEK articulation interfaces to support the complex multidirectional motion required for such joints under physiological loadings. In view of the failure of polymer on polymer bearing couples to achieve clinical success in weight bearing joint arthroplasty devices, and the lack of knowledge of wear and other performance characteristics of PEEK on PEEK combinations under physiological loadings and kinematics, there has been no known motivation in the art to develop implants comprising PEEK on PEEK articulation interfaces for these rigorous applications. To this end, conventional approaches to bearing couplings teach that having surfaces of the same material bearing and articulating against one another, i.e. self-mating bearing couples, will not lead to acceptable performance. In particular, if the same metal material is used for both bearing surfaces galling of one or both of the metal surfaces is likely to occur under sustained loading. Similarly, where the bearing surfaces are of the same polymer material, there is the problem of surface adhesion under high loads, e.g. beyond the yield point of the polymer material, that has led to use of articulating load bearing members of different materials in artificial load bearing joints, such as metal on UHMWPE in artificial intervertebral joints.
Thus, there is a need for weight bearing total joint arthroplasty devices having excellent strength, biocompatibility, biodurability, friction and wear characteristics for high performance, longer life and lower risk of adverse responses such as particulate induced inflammation and osteolysis. There is also need for such devices having articulating surfaces that do not produce potentially harmful metallic wear particulate. Ideally, known problems of using polymeric articulation surfaces, such as higher failure rates and the increased wear associated with strain hardening caused by multidirectional motion, could also be overcome. Such devices are needed for applications requiring conforming bearing surface, such as an acetabular cup for a hip joint, and also for high stress, non-conforming contact applications such as in a knee joint.
There is also an unmet need for devices that meet these requirements while also being substantially radiolucent for improved imaging of the affected area. Ideally, such devices would also have a modulus of elasticity closer to the adjacent bone tissue to minimize the adverse effects of stress shielding on the adjacent bone. There is a further need to reduce the number of components in such total joint arthroplasty devices so as to provide fewer modes of failure, to reduce parts inventory and simplify manufacturing and assembly. Such devices should also be readily sterilized using conventional radiation or steam sterilization techniques without causing oxidation and associated adverse effects. Ideally such devices could be provided for the major weight bearing joints in a range of sizes required to serve the full patient populations for various degenerative joint conditions.